Hitherto, devices for receiving externally transmitted electromagnetic waves, generating electromagnetic waves which include digital information indicating status of the device and digital information held (stored) therein, and externally transmitting the information have been known (hereinafter these devices are called “information transmitting devices”).
One example of these devices is a wireless position indicator used in an electromagnetic induction type digitizer (see Japanese Unexamined Patent Application Publication Nos. 3-189716 and 7-175572).
The above-mentioned position indicators include a resonant circuit tuning with externally transmitted electromagnetic waves; a digital information generator for generating digital information including a binary code of 2 bits or more, the digital information corresponding to an operation and/or being stored in a memory or the like in advance; and a resonance characteristic controller for changing a characteristic of the resonant circuit in accordance with any one of ‘0’ and ‘1’ of the binary code in synchronization with at least part of the electromagnetic waves. By allowing an information receiver side to recognize the change in the characteristic of the resonant circuit, digital information is transmitted.
Further, in this type of information transmitting device, by providing a power extracting unit for extracting power for driving each component, that is, the digital information generator and the resonance characteristic controller, from an induced voltage which is generated in the resonant circuit due to externally transmitted electromagnetic waves, a power supply such as a battery is not necessary.
The above-described resonance characteristic controllers may be classified into two types. In one of them, an element is connected to an element of the resonant circuit in accordance with any one of ‘0’ and ‘1’ of a binary code so as to change frequencies of transmitted electromagnetic waves. In the other, by shorting across an element of the resonant circuit in accordance with any one of ‘0’ and ‘1’ of a binary code, electromagnetic waves are generated (ON-state) or not generated (OFF-state).
The latter resonance characteristic controller has the following advantages. That is, an element other than an element of the resonant circuit is unnecessary, an information receiver side should detect only a reception signal level at predetermined timing, and detection of a change in frequencies of electromagnetic waves, which needs a filter and a phase detector, need not be performed.
On the other hand, the latter resonance characteristic controller has the following problem. That is, power cannot be extracted from the resonant circuit while the resonant circuit is shorted, and thus a power supply voltage supplied from the power extracting unit to each component is likely to become unstable.
In particular, when the digital information generator is configured to detect analog information corresponding to an operation represented by a continuous amount, such as a writing pressure, and then to convert the-analog information to digital information (binary code), a digit of the binary code increases or decreases in accordance with changes in analog information. If ‘0’ or ‘1’ in a code significantly varies, the power supply voltage becomes very unstable. Accordingly, analog information cannot be stably detected and detection accuracy decreases.